Homeostasis

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Homeostasis is the tendency of a cell or the whole organism to maintain a state of balance. Molecules pass into and out of the cell to maintain this balance. The cells of the body constantly adjust to preserve a balance of fluids, temperature, oxygen, electrolytes, and nutrients.

Electrolytes are compounds made of charged particles called ions. These ions can conduct electrical current in water or in the cytoplasm of the cell. A positive charge, or cation, creates an acid. A negative charge, or anion, creates a base. The pH of a fluid is a measurement of how much acid or base is present. Each body tissue has a normal pH. The cells do not function properly if the normal pH is not maintained for the area of the body (Figure 8-2). Different electrolytes also have specific functions, as shown in Table 8-1.

Figure 8-2

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Table 8-1   -- Electrolytes of the Body

The structures of the cell.

pH of the body. The body must maintain normal pH levels to function properly.

Anatomy and Physiology The human body, like all living organisms, has four basic properties of life:    •  Reception is the ability of the organism to control its actions and respond to changes in the environment.    •  Metabolism is the process of taking in and using nutrients to produce energy and growth.    •  Reproduction is the ability to reproduce offspring to continue the species.    •  Organization divides the organism into distinct parts to perform these functions. The two major types of study of the human body are called anatomy and physiology. Anatomy is the study of body structures and their location. Body structures are organized on five levels:    •  Cells are the smallest unit of life.    •  Tissues are combinations of similar cells.    •  Organs are collections of tissues working together to perform a function.    •  Body systems consist of organs that work together to provide a major body function.    •  Organisms are the beings that result when the body systems work together to maintain life. Cell Structure The major structures, called organelles, of the cell are shown in Figure 8-1. These structures are as follows:    •  The nucleus controls the activity of the cell and directs reproduction.    •  The cytoplasm is a semifluid material that surrounds the cell parts and transports chemicals and nutrients within the cell.    •  Mitochondria produce the energy used for cellular processes.    •  The cell membrane surrounds the cell and controls which substances enter and leave the cell.    •  Lysosomes help to break down, or digest, molecules.    •  Ribosomes attached to the endoplasmic reticulum work to produce protein for the cell structures.    •  The Golgi apparatus helps to transport proteins made by the ribosomes out of the cell by making glycoproteins. Figure 8-1

Ion	Function
Cations(+)
Sodium (Na+)	Controls water distribution by increasing ability of fluid to pass through cell membrane
Potassium (K+)	Maintains fluid balance, promotes growth of cells, nerve conduction, muscle contraction, and heart activity
Calcium (Ca++)	Controls neuromuscular irritability, muscle contraction, blood clotting, building bones and teeth
Magnesium (Mg++)	Maintains neuromuscular system, activates enzymes, regulates level of phosphorus
Hydrogen (H+)	Needed for cell and enzyme functions, binding of oxygen to hemoglobin
Anions (−)
Bicarbonate (HCO3−)	Maintains acid-base balance
Phosphate (HPO4=)	Maintains fluid and acid-base balance
Chloride (C−)	Maintains fluid balance
Sulfate (SO4=)	Maintains fluid balance

Tissue Types

There are four main groups of tissue in the body (Figure 8-3).

	•	Epithelial tissue covers the body, forms glands, and lines the surfaces of cavities and organs.
	•	Connective tissue, formed by a protein, includes soft tissue such as fat and blood cells and hard tissues such as bones, ligaments, and cartilage.
	•	Muscle tissue, made of protein fibers, has the unique property of shortening in length to produce movement.
	•	Nervous tissue, composed largely of specialized cells called neurons, is found in the eyes, ears, brain, spinal cord, and peripheral nerves. Nervous tissue transmits communications.

Figure 8-3Tissues of the body. A, Epithelial. B, Connective. C, Muscle. D, Nervous. (A, B, and D, Courtesy Ward's Natural Science Establishment, Rochester, NY.) 600 387 600 387 79 51

Body Systems

The study of the functions of the body is called physiology. Functions are studied according to body systems. A body system is a group of related organs that together accomplish functions necessary to maintain and support life. The 12 body systems are as follows:

	•	The integumentary system covers the body and protects other body systems.
	•	The cardiovascular system transports oxygen and nutrients to all body parts and removes waste products.
	•	The circulatory system includes the blood and lymph that move throughout the body.
	•	The respiratory system exchanges gases between the air and blood.
	•	The muscular system allows the body to move and controls movements within the body.
	•	The skeletal system provides body support and protection.
	•	The digestive system processes food and eliminates food waste.
	•	The urinary system filters the blood and removes liquid wastes.
	•	The endocrine system coordinates body activities through hormones.
	•	The nervous system regulates the environment and directs the activities of other body systems.
	•	The sensory system perceives the environment and sends messages to and from the brain.
	•	The reproductive system provides for human reproduction.

The following 12 chapters discuss the body systems in more detail.

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